Provide macros to replace usages of `assert`.  A true `assert` is
punishing as a library.  Instead we should do our best to not crash.

GIT_ASSERT_ARG(x) will now assert that the given argument complies to
some format and sets an error message and returns `-1` if it does not.

GIT_ASSERT(x) is for internal usage, and available as an internal
consistency check.  It will set an error message and return `-1` in the
event of failure.

Fix uninitialized stack memory and NULL ptr dereference in stash_to_index

Caught by static analysis.

The checkout code didn't iterate into a subdir if it didn't match the
pathspec, but since the pathspec might match files in the subdir we
should recurse into it (In contrast to gitignore handling).

Fixes #5089

Honor GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH for all checkout types 

We were previously applying the pathspec filter for the baseline
iterator during checkout, as well as the target tree.  This was an
oversight; in fact, we should apply the pathspec filter to _all_
checkout targets, not just trees.

Add a helper function to set the iterator pathspecs from the given
checkout pathspecs, and call it everywhere.

The checkout::index::can_disable_pathspec_match test attempts to set a
path filter of `test11.txt` and `test12.txt`, but then validates that
`test10.txt` and `test11.txt` were left unmodified.  Update the test's
path filter to match the expectation.

docs: add documentation for our coding style

MSVC: Enable Control Flow Guard (CFG)

git__hexdump: better mimic `hexdump -C`

This feature requires Visual Studio 2015 (MSVC_VERSION = 1900) or later. As the
minimum required CMake version is currently less than 3.7, GREATER_EQUAL is not
available to us and we must invert the result of the LESS operator.

Feature: Allow blame to ignore whitespace change

deps: ntlmclient: use htobe64 on NetBSD too

For years, we've repeatedly had confusion about what our actual coding
style is not only for newcomers, but also across the core contributors.
This can mostly be attributed to the fact that we do not have any coding
conventions written down. This is now a thing of the past with the
introduction of a new document that gives an initial overview of our
style and most important best practices for both our C codebase as well
as for CMake.

While the proposed coding style for our C codebase should be rather
uncontroversial, the coding style for CMake might be. This can be
attributed to multiple facts. First, the CMake code base doesn't really
have any uniform coding style and is quite outdated in a lot of places.
Second, the proposed coding style actually breaks with our existing one:
we currently use all-uppercase function names and variables, but the
documented coding style says we use all-lowercase function names but
all-uppercase variables.

It's common practice in CMake to write variables in all upper-case, and
in fact all variables made available by CMake are exactly that. As
variables are case-sensitive in CMake, we cannot and shouldn't break
with this. In contrast, function calls are case insensitive, and modern
CMake always uses all-lowercase ones. I argue we should do the same to
get in line with other codebases and to reduce the likelihood of
repetitive strain injuries.

So especially for CMake, the proposed coding style says something we
don't have yet. I'm fine with that, as the document explicitly says that
it's what we want to have and not what we have right now.

sysdir: remove unused git_sysdir_get_str

Fix typo causing removal of symbol 'git_worktree_prune_init_options'

Commit 0b5ba0d7 replaced this function with an "option_init"
equivallent, but misspelled the replacement function. As a result, this
symbol has been missing from libgit2.so ever since.

pack: Improve error handling for get_delta_base()

repo::open: ensure we can open the repository

examples: additions and fixes

merge: cache negative cache results for similarity metrics

Update the test cases to check the `git_repository_open` return code.

add example for git commit
fix example for git add
add example for git push

Handle repository format v1

CMake: backend selection streamlining

refdb_fs: remove unused header file

This makes get_delta_base() return the error code as the return value
and the delta base as an out-parameter.

This change moves the responsibility of setting the error upon failures
of get_delta_base() to get_delta_base() instead of its callers. That
way, the caller chan always check if the return value is negative and
mark the whole operation as an error instead of using garbage values,
which can lead to crashes if the .pack files are malformed.

patch: correctly handle mode changes for renames

gitignore: clean up patterns from old times

README.md: update build matrix to reflect our latest releases

We're currently doing unnecessary work to auto-detect backends even if
the functionality is disabled altogether. Let's fix this by removing the
extraneous FOO_BACKEND variables, instead letting auto-detection modify
the variable itself.

Release v1.0